Sand from the Rocks for Concrete Production

Aggregates Processing

Sand from the Rocks for Concrete Production

New Type of Crushed Sand to replace natural Sand in Concrete Production
The availability of natural sand for concrete production is facing challenges, while the so-called waste stockpiles at aggregate crushing areas are causing problems for producers. This means that the industry has a huge need to solve this challenge by finding suitable technology for usable crushed sand production.
(ed. WoMaMarcel - 24/9/2015)
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From a concrete technology point of view, those findings can be explained by the adsorption of the free water on a larger total surface area of a finer grading, thus leaving less water available for the lubrication of the cement paste itself. The recent findings have also indicated that varying the properties (flakiness) of coarser crushed sand fractions (0.125/2 mm and 2/4 mm) has much less relative impact on fresh concrete properties, and that the other fine particle (≤ 0.125 mm) characteristics, such as shape, surface texture and mineralogy, could have a similar effect as grading (specific surface).

Further Improvement in Concrete Properties

The current state-of-the-art for crushed sand production is VSI shaping and wet or dry classification to reduce the total fines content. However, it is our belief – and our findings indicate – that further important improvement in concrete properties can be achieved if the properties of the very fine part of the crushed sand (≤ 0.125 mm) are modified (engineered).

Two tasks then have to be solved, in order to see if our hypothesis is valid also when applied to industrial scale production. The first task includes the concrete technology side, which would require a complete understanding of how and which fine particle properties affect fresh concrete properties. The second task involves finding an industrial solution that would render a controlled modification and optimization of the filler part properties possible also at aggregates quarries.

Otherwise, our findings would be only of scientific importance with limited practical relevance. What seemed to be a difficult task at the beginning turned out to be possible with the equipment readily offered by Metso Minerals. This is because if the fine (≤ 0.125 mm) part of the crushed sand is separated into different fractions by Metso’s static air-classifiers and stored into silos, we just need to find the best way of combining them afterwards.

Moreover, two-stage dry air-classification is a concept that is already used today, providing in total four dry filler fractions to be mixed back either before or during the concrete production. The static air-classifiers are perfectly suited for aggregate operations, since they do not have any moving parts; the inside of their chambers is lined with ceramic lining, assuring very low wear costs even with highly abrasive feeds.

At the same time, they can be designed to accommodate the throughput necessary for most aggregate operations. In addition, they do not have the problems usually associated with the wet classification process (washing), i.e. space and environmental issues from de-watering ponds and operation problems in places where negative temperatures are reached in the winter time.

Developing a future Sand Solution

In order to proceed with the experiments to further elaborate the proposed hypo-thesis, great effort and care were taken in the preparation of a set of model materials (crushed fillers) that represent the complete range of local geological variety in Scandinavia. This involved collecting 4/22 mm crushed rock samples from 10 different quarries. Further processing included another step of Barmac VSI crushing to generate fines and screening of 0/4 mm crushed sand. Special care was taken to ensure that all the fines were generated only in this strictly controlled way.

Previous research findings within the COIN project and elsewhere had proved that the geometrical properties (such as shape) of the fine particles ≤ 0.125 mm can be affected by the crushing procedures applied. For example, an increase in VSI tip speed demonstrated an improvement in the properties of particles all the way down to and including the filler (≤ 0.125 mm or 120 mesh) sizes; moreover, those improvements proved the introduction of measurable changes in the fresh concrete properties.

After VSI crushing, the 0/4 mm crushed sand material was further split in two fractions – 2/4 mm and 0/2 mm. Almost 9 tons of the 0/2 mm fractions were then sent to Metso’s air-classification laboratory in Lebanon, Pennsylvania, USA, for controlled removal of filler and alteration of filler grading. This means that as much material below 0.125 mm as possible was removed and further split into fractions 0.063/0.125, 0.020/0.063 and 0/0.020 mm.

Industrially, this could be achieved by connecting a gravitational-inertial (GI) air-classifier in a row with two centrifugal (C) classifiers. This is illustrated in the flow chart in Fig. 4. As mentioned, already today there are aggregate operations that have two Metso static air-classifiers (GI and C) connected in a row to produce multiple different fine filler fractions.

Fig. 4: Metso Minerals air-classification lab in Lebanon, PA, USA (above) and flow sheet of the experiments carried out.
Metso Minerals static air-classifiers, C - Centrifugal, GI - Gravitational Inertial, G - Gravitational.

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